1. Field of the Invention
The present invention relates to the adsorptive separation of gases.
More particularly, the invention relates to a method of separating nitrogen by adsorption from a gas mixture which contains, in addition to the nitrogen component, at least oxygen as another component.
Still more specifically, the invention relates to a method of effecting the aforementioned separation by alternative adsorption and desorption of carbonaceous molecular-sieve coke.
2. The Prior Art
The basic principle of effecting gas separation by adsorption is known. To obtain a nitrogen-rich product gas it is customary to pass a gas mixture through a body of molecular-sieve coke in a reactor vessel or adsorber until the product gas issuing from the body is found to contain an undesirably high oxygen contant of, e.g., 4%. This is indicative of the fact that the molecular-sieve coke has adsorbed oxygen to the limit of its capacity and must now be desorbed before it is able to perform further useful work. Consequently, when a rise of the oxygen content in the product gas beyond a certain threshold value is detected, the stream of gas mixture--usually air--is diverted to a second reactor vessel containing another body of molecular-sieve coke which has been previously desorbed, as by, e.g., evacuation of the vessel. The previous separation procedure is now repeated in the second vessel and during the time the molecular-sieve coke--hereafter called "adsorbent" for convenience--of the first vessel is desorbed.
The separation phase during which the adsorption takes place, is generally effected at a pressure of about 1-10 bar; the flow of starting gas (i.e., of the incoming gas mixture) through the adsorber may take place at constant pressure or at steadily increasing pressure. The subsequent desorption is effected by pressure reduction in the adsorber down to 1 bar or to below 100 Torr. The product gas may during the separation phase be vented from the adsorber as soon as the separation begins or else such venting may be delayed until pressure in the adsorber rises to between 3-20 bar, preferably to 4-10 bar. Venting of the product gas then starts only when the preselected pressure level is reached; once venting begins the pressure level may be maintained or it may be reduced.
It has been found to be advantageous to effect a pressure equalization between the two adsorbers at their inlets and outlets before the stream of starting gas is switched to the newly-desorbed adsorber; when this is completed, the stream of starting gas is then made to pass through the desorbed adsorber and the previously used fully loaded adsorber is now in turn desorbed.
In connection with this procedure, however, a problem has been observed which has heretofore defied a solution. It is found that when starting gas passes through the newly desorbed adsorber, the product gas which is initially obtained is not nitrogen of the expected purity--but nitrogen which has a slightly elevated oxygen content of, e.g., about 0.25 vol/%. The expected degree of purity, e.g., about 0.1 vol/% of oxygen in the product gas, is reached only after the product gas has been issuing from the adsorber for a period of about 10 seconds. But this oxygen differential, while perhaps fairly slight when considered overall, has heretofore stood in the way of all efforts to obtain a product gas having the low average oxygen content which it should be theoretically possible to achieve.